1. Field of the Invention
The present invention relates to an antenna apparatus, and more particularly, to an antenna apparatus used for receiving an electric wave for the GPS (Global Positioning System), a satellite radio, or the like.
2. Description of the Related Art
For example, various types of antenna apparatuses are developed as an antenna apparatus for in-car GPS system which is popular as a positioning system and, as an antenna apparatus used for a satellite radio or the like of in-car type or emplacement type for household use or the like which is in practical use in the United States (for example, see JP2005-110007A, JP2004-72320A, and JP2004-228357A).
In such an antenna apparatus, for example, a circuit board 103 is attached on a back surface of an antenna element 102 having a patch-type receiving face 101 which receives the electric wave as in the antenna apparatus 100 which is shown as an example in FIG. 5. An amplifier circuit (omitted from the drawing) to amplify the signal which is input from the antenna element 102 is formed on the opposite surface of the antenna element 102 of the circuit board 103. The surface in which the amplifier circuit is formed is covered with an approximately box-shaped shield cover 104 which shields the extraneous interfering wave. In FIG. 5, the receiving face 101 is expressed thicker than the actual receiving face 101.
The input pin 105 is penetratingly inserted in the antenna element 102 and the circuit board 103 so as to be vertical to the antenna element 102 and the circuit board 103, and one end of the input pin 105 is electrically connected to the receiving face 101 of the antenna element 102 by soldering. The other end of the input pin 105 is soldered to the amplifier circuit on the circuit board 103 and is electrically connected thereto, and an input unit 103a is formed there. The input pin 105 inputs the electric wave signal, which is received by the receiving face 101 of the antenna element 102, in the amplifier circuit of the circuit board 103.
Moreover, a coaxial cable 106 is inserted inside the shield cover 104. A core wire 106a of the coaxial cable 106 is soldered to the amplifier circuit on the circuit board 103 and is electrically connected thereto, and the connected portion composes an output unit 103b of the amplifier circuit. The coaxial cable 106 supplies the driving power to the amplifier circuit via the core wire 106a, and outputs the signal which is received by the antenna element 102 and amplified by the amplifier circuit.
Furthermore, the shield cover 104 assumes the function of the GND, and the shield cover 104 is to have the GND potential via the tongue 104b due to the tongue 104b, which is formed by being bent in the coaxial cable 106 side from the basal face 104a of the shield cover 104, being soldered and electrically connected to an external conductor 106b of the coaxial cable 106. Accordingly, the amplifier circuit is to be grounded via the shield cover 104.
In recent years, downsizing of such antenna apparatus is being attempted. When the apparatus is downsized, the input unit 103a and the output unit 103b of the amplifier circuit of the circuit board 103 approximate one another. When the input unit 103a and the output unit 103b approximate one another as described above, an isolation of the signal input/output is degraded and there is a problem that the crosstalk occurs between the input unit 103a and the output unit 103b. 
The crosstalk occurs in the air or in the circuit board between the input unit 103a and the output unit 103b. Because of the crosstalk, a portion of the signal which is input in the amplifier circuit from the antenna element 102 via the input unit 103a and which is amplified is leaked to the input unit 103a from the output unit 103b and the positive feedback occurs to the circuit. In a worst situation, a standing wave is generated in the amplifier circuit, and an abnormal oscillation phenomenon in which signal strength of the particular wave length becomes strong occurs.
Therefore, closer the distance between the input unit 103a and the output unit 103b of the circuit due to the downsizing of the apparatus, greater the degree of crosstalk to the input unit 103a from the output unit 103b, and there is a problem that the generated abnormal oscillation becomes unusually strong. Further, the characteristic of the crosstalk also changes according to the wave length of the electric wave which is received by the antenna element 102. In the range of several hundred MHz which is the frequency of television, it is known that the contribution of the crosstalk which transmits in the air is greater than the contribution of the crosstalk in the circuit board.